The present invention relates to an evaluation circuit for a capacitive sensor.
In order to produce electrical signals as a function of different physical variables, capacitive sensors are known in which the capacitance of a capacitor is dependent on the specific physical variable. The capacitance values are frequently within a range of 1 pF to 10 pF and thus difficult to evaluate. In those cases in which the capacitances of two capacitors vary in opposite directions, simpler circuits have become known, but the provision of so-called differential capacitors is not always possible.
From European Patent EP 0 175 362 A2 a capacitive sensor for the control of a windshield wiper is known in which the wetness-dependent capacitance is part of an oscillator circuit. Due to their low capacitance, however, such circuits oscillate only with very high frequency. This, in its turn, results in susceptibility to disturbance through radiations and losses in the windshield.